Conventionally, polyorganosiloxane compositions curing into silicone rubber have been well known, and are widely used as a potting material, a coating material, a molding material for shaping, injection molding and so on, and a covering material in an electric and electronic field, and optical and optoelectronics, sensor, architecture fields, utilizing its excellent properties such as weather resistance, heat resistance, electrical insulation, hardness, mechanical strength, elongation and so on. Above all, a polyorganosiloxane composition which is cured by an additional reaction is cured rapidly by appropriate heating and a corrosive substance and the like are not emitted at a time of the curing, so that a use in each of the fields spreads.
As a technique of imparting flame retardancy to such an additional reaction curing-type polyorganosiloxane composition, conventionally, addition of a platinum-based metal compound has been performed. However, when it is intended to obtain sufficient flame retardancy, an addition amount of the platinum-based metal compound is to be increased, so that there has been a problem that transparency becomes poor, such as discoloration (yellowing) of a cured product. Further, from the viewpoint of a material cost, it is required that the addition amount of the platinum-based metal compound is reduced as small as possible.
In order to improve the flame retardancy, methods (refer to Patent Reference 1 (U.S. Pat. No. 3,652,488) and Patent Reference 2 (U.S. Pat. No. 3,635,874), for example.) of compounding carbon black or titanium oxide, a method (refer to Patent Reference 3 (JP-A Hei 11-140320), for example.) of compounding a flame retardant other than a platinum-based compound, such as phosphoric acid ester or phosphite, and the like are carried out.
However, in the methods described in Patent Reference 1 and Patent Reference 2, not only decreases in other physical properties easily occur, but also a transparent cured product cannot be obtained by coloring of compositions, so that it is difficult to use for a sealant for a light emitting device and optical materials such as a lens and a cover, or the like and thereby a use is limited. Further, there has been a defect in which flame retardancy decreases with time.
Further, in a method described in Patent Reference 3, compatibility with a flame retardant is not sufficient depending on the type of a base polymer, so that there have been problems that the flame retardant easily separates, a cured product easily discolors (yellows) with time, and the like.
Moreover, as curable silicone compositions, a silicone composition has been proposed, as a curing silicone composition, which contains an alkenyl group-containing organopolysiloxane composed of a dialkylpolysiloxane having two or more alkenyl groups, on average, in a molecule and having a predetermined viscosity and an organopolysiloxane having a tetrafunctional siloxane unit and containing a alkenyl group at a predetermined rate as a main component, an organohydrogenpolysiloxane having a tetrafunctional siloxane unit and containing a silicon atom-bonded hydrogen atom at a predetermined rate, and a hydrosilylation reaction catalyst, and having a hardness of 30 or more and 80 or less and an elongation of 50% or more (in conformity with JIS K6253), or having a hardness of 75 or less and an elongation of 35% or more (for example, refer to Patent Reference 4 (Japanese Patent No. 5475295) and Patent Reference 5 (Japanese Patent No. 5568240).).
However, the compositions described in Patent Reference 4 and Patent Reference 5 have been excellent in mechanical strength (hardness and elongation) of cured products, but have not had sufficient flame retardancy.